1. Field of the Invention
The invention relates generally to the fields of materials and nanotechnology. More particularly, the invention relates to at least one fiber on an electrically conductive interconnect located on an insulating surface of a substrate. Specifically, a preferred implementation of the invention relates to a plurality of individually electrically addressable vertically aligned carbon nanofibers that are grown by DC plasma enhanced chemical vapor deposition on the electrically conductive interconnect. The invention thus relates to vertically aligned carbon nanofibers of the type that can be termed individually electrically addressable.
2. Discussion of the Related Art
Nanostructured graphitic carbon-based materials are among the most promising candidates for microfabricated cold cathode FE devices such as parallel electron sources for electron microscopy, electron beam lithography and FE flat panel displays. The FE properties of the VACNF have been studied in detail and demonstrate that the VACNFs behave as robust FE cathodes. In order to make full use of the VACNF in FE devices a way to individually electrically contact the base of the VACNF is required. This allows a way to precisely control the emitted current through a single VACNF during device operation.
VACNFs are produced in a DC plasma enhanced chemical vapor deposition (PECVD) process. Aspects of this synthesis process require an electrostatic potential to be placed on the substrate material from which the VACNFs are to be grown. As a consequence, it is not possible to produce individual VACNFs on substrates that are not electrically conductive. This presents several problems for creating arrays of VACNFs that are individually electrically addressable. A means of creating such arrays on insulating substrates is essential for realizing practical VACNF-based FE devices. However, no process has currently been reported that can achieve this goal.
Electrical and electrochemical methods for characterizing small volumes are a critical need in many fields of study including semiconductor manufacture (device characterization and defect detection), materials science (composition, corrosion, and interface characterization), and biology and biotechnology (intra- and extracellular molecular probing, electrophysiology). To create probing devices with improved spatial resolution that can accurately characterize these mesoscale phenomena, it is necessary to fabricate probes with nanoscale electrically (and electrochemically) active probe tips. Moreover, to achieve the highest sensing resolution possible, fabrication process for such probes should offer a means to deterministically control the active surface area (and thus probing volume) at the tip of the device.
In recent years, a large research effort has focused on the development of microelectrode electrochemical probes for probing small volumes at fluid/material interfaces and within and around biological cells. The promise of these electrodes lies in their fast response times (ms time scale), high mass sensitivity (zeptomole, i.e. 100–1000 molecules), small size, large linear dynamic range (up to 4 orders of magnitude) and selectivity, and because molecules of interest can be followed without the need for derivatization as is necessary in fluorescence microscopy. The microelectrodes reported to date for small volume analysis typically consist of a linear carbon nanotube bundle surrounded by an insulating layer of glass. Macroscopic carbon bundles are placed into glass capillary tubes, which are then pulled down to microscale dimensions (0.5 to 10 μm). Microscopic inspection allows the cleaving of the pulled capillary in the vicinity of the entrained carbon bundle, providing a conductive carbon tip, surrounded by an insulating sheath of glass. In electrolyte systems, the resultant microelectrode can be placed in close proximity to a region of interest, and used to analyze the local microenvironment for a large variety of electroactive species, such as galvanic corrosion species (in materials science applications) and many biological compounds. For easily oxidizable substances, i.e., catecholamines, indoleamines, oxygen, and doxorubicin, the native carbon electrode surface is sufficient for electrochemical analysis and these substances have been readily detected in or near the surface of single cells. In fact these electrodes, at present, are one of only a few available techniques to measure the exocytotic release of neurotransmitters from single cells.
The small size of these electroanalytical probes provides obvious advantages in terms of application and placement of the electrode at or within very small volumes and for measuring the spatial and temporal dynamics of electrochemically active species within small scale systems. The probing volume of an individual probe is defined by the electrochemical boundary layer of the electrode, which is typically a region consisting of a 10 nm radial distance from the electrode surface. Therefore, the small electroactive surface area of an electrochemical microelectrode provides for the ability to probe very small regions of interest, and to obtain quantitative information of a measured molecular species within extremely localized volumetric regions. The small size also provides many advantages in terms of the electrochemistry involved. Perhaps most significant is that the reduction in electrochemically active surface area results directly in a reduction of the double layer capacitance that exists at the interface between any electrode/electrolyte system. This improves the ratio of faradaic to non-faradaic (or ‘charging’) current at the electrode and thus increases the signal to noise ratio of most electroanalytical techniques. The reduction in capacitance also provides for more rapid electroanalysis to be conducted within the region of interest due to the resultant decrease in background or charging current that plagues larger electrode systems. The reduced electrode surface area also reduces the overall currents generated between electrodes during electroanalytical events, and thus greatly reduces the amount of potentially interfering products that are generated during electroanalysis that might otherwise have deleterious effects on the local environment. Thus, electroanalysis at ultramicroelectrodes provides for the ability to monitor molecular events, while generating minimal perturbation to the system during the measurement.
While microelectrodes reported in the literature have allowed unprecedented spatial and temporal measurements to be made within small volumes, the electrodes themselves are still rather large compared to the size of many targeted systems (i.e. biological cells and nanoscale material interfaces). They are also monolithic, single electrode components, therefore spatial mapping of species in and around a system requires physically moving either the analyzed system or the electrode. The fabrication process of individual probes is also quite arduous, requiring the selection of carbon fiber bundles, placement in a capillary, pulling the capillary, cleaving, polishing, and interconnecting to macroscopic analysis instruments. As a consequence, there is a high degree of variation from probe to probe, due to differences in the fabrication process, in addition to the electrochemical behavior of the raw materials.